The present invention generally relates to noise reduction circuits, and more particularly to a noise reduction circuit which reduces pulse noise in reproduced audio signals by holding previous values or predicted value thereof in a helical scan type magnetic recording and reproducing apparatus which uses rotary heads to record and reproduce on and from a magnetic tape frequency modulated audio signals which are obtained by frequency-modulating carriers by audio signals.
Conventionally, in order to record and reproduce an audio signal in a helical scan type magnetic recording and reproducing apparatus (hereinafter referred to as a VTR) with a high quality, it is known to record and reproduce on and from a magnetic tape a frequency modulated audio signal (hereinafter simply referred to as an FM audio signal) which is obtained by frequency-modulating a carrier by the audio signal. The audio signal which is to be recorded is encoded in a first noise reducing part so that noise can be reduced at the time of a reproduction. A high frequency component of the encoded audio signal is emphasized in a pre-emphasis circuit so that high frequency noise can be reduced, and the audio signal from the pre-emphasis circuit is supplied to a frequency modulator. An output FM audio signal of the frequency modulator is amplified and is recorded on oblique tracks of the magnetic tape by a pair of rotary heads which alternately form the oblique tracks. At the time of the reproduction, reproduced FM audio signals from the rotary heads are alternately supplied to a switching circuit. The switching circuit switches over so as to selectively pass the output of the rotary head which is scanning the oblique track of the magnetic tape, and thus, a continuous reproduced FM audio signal is obtained from the switching circuit. The reproduced FM audio signal from the switching circuit is supplied to a frequency modulator and an envelope detection circuit. A reproduced audio signal obtained from the frequency demodulator is supplied to a hold circuit wherein the signal is held during a time period in which a hold signal is supplied to the hold circuit from an adding circuit which adds outputs of first and second hold signal generating circuits.
The reproduced audio signal from the frequency demodulator includes large noise when (a) the reproduced FM audio signals from the rotary heads are switched in the switching circuit, (b) when an envelope level of the reproduced FM audio signal decreases due to a signal dropout caused by scratches, dust particles and the like on the magnetic tape, (c) when the envelope level of the reproduced FM audio signal decreases due to tracking error and the like of the rotary heads, and (d) when playing a magnetic tape which is not recorded with an FM audio signal. In the case (a), pulse noise is generated in the reproduced audio signal because a continuity in a waveform of the reproduced FM audio signal is distorted when the reproduced FM audio signals are switched in the switching circuit due to causes such as a difference in tensions of the magnetic tape at the time of the recording and at the time of the reproduction. In the cases (b), (c) and (d), noise is generated essentially due to the absence of the FM audio signal, and in these cases, the noise is generated for long time periods compared to a time period in which the pulse noise is generated. Generally, the time period in which the noise is generated in the case (c) is longer than the time period in which the noise is generated in the case (b) and is shorter than the time period in which the noise is generated in the case (d).
The noise described above is reduced in the hold circuit and a muting circuit. An output signal of the hold circuit is supplied to a de-emphasis circuit wherein the high frequency component of the signal emphasized at the time of the recording is attenuated. An output signal of the de-emphasis circuit is given a level expanding characteristic complementary to that at the time of the recording in a second noise reducing part and is thereafter supplied to the muting circuit. A muting signal generating circuit generates a muting signal which has a high level when an output detection signal of the envelope detection circuit is smaller than a predetermined level for over a certain time period and holds the high level until a specific time period elapses from a time when the output detection signal becomes larger than the predetermined level. The muting circuit performs a muting operation only during a high level period of the muting signal. When the reproduced FM audio signals from the rotary heads are switched in the switching circuit, the envelope level of the output reproduced FM audio signal is larger than the predetermined level, and the level of the muting signal remains at the low level. For this reason, the muting circuit does not perform the muting operation, and an output reproduced audio signal waveform of the mtuing circuit is the same as that of the hold circuit.
On the other hand, when the noise is generated for a relatively long time period due to the signal dropout or the decrease in the envelope level of the reproduced FM audio signal, a hold signal which has a low level only during a time period in which the hold signal is generated based on the output detection signal of the envelope detection circuit is supplied from the second hold signal generating circuit to the hold circuit via the adding circuit. Accordingly, during the time period in which the noise is generated, the reproduced audio signal which is held at the signal level immediately before the noise is generated is passed through the de-emphasis circuit and the second noise reducing part, and is supplied to the muting circuit wherein the signal is muted during a hold time period and a certain time period immediately thereafter.
In the VTR having the construction described heretofore, a noise reduction circuit for reducing pulse noise in the reproduced audio signal is constituted by the hold circuit, the first hold signal generating circuit and the adding circuit described above. In one example of the conventional noise reduction circuit, the pulse noise can be eliminated by the hold operation of the hold circuit as will be described later on in the specification in conjunction with the drawings, however, there is a problem in that an error corresponding to a change in the signal during the hold time period remains in the reproduced audio signal. This error becomes large as a slew rate of the reproduced audio signal becomes high.
In another example of the conventional noise reduction circuit, an interpolation signal generating circuit is provided within the hold circuit as will be described later in conjunction with the drawings. According to this conventional noise reduction circuit, the error which remains in the reproduced audio signal during the hold time period is extremely small when the slew rate of the reproduced audio signal is low. Furthermore, even when the slew rate of the reproduced audio signal is high, the error which remains in the reproduced audio signal is small compared to that of the conventional noise reduction circuit described before. However, when the slew rate of the reproduced audio signal is high, there is a problem in that it is impossible to make the error which remains in the reproduced audio signal extremely small. On the other hand, normally, in addition to the pulse noise, the reproduced audio signal in most cases includes a high frequency noise component which constantly has a low level. Because the high frequency noise component is included in the reproduced audio signal, a slope which is predicted in the interpolation signal generating circuit becomes inconsistent. When the slew rate of the reproduced audio signal is high, the error which remains in the reproduced audio signal is small compared to the conventional noise reduction circuit described before which performs the simple hold operation, even when the inconsistency is introduced in the slope prediction. However, when the slew rate of the reproduced audio signal is low and the inconsistency is introduced in the slope prediction, there is a problem in that the error which remains in the reproduced audio signal becomes larger than that of the conventional noise reduction circuit described before which performs the simple hold operation.